error[E0369]: binary operation `+` cannot be applied to type `&std::string::String`
  --> $DIR/str-concat-on-double-ref.rs:14:13
   |
LL |     let c = a + b;
   |             ^^^^^
   |
   = note: an implementation of `std::ops::Add` might be missing for `&std::string::String`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0369`.